Looks you ignore close(socket filehandle), i mean like this :

sock($server);
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sorry..typos

close $server;
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Have a look at code below :

use strict;
use IO::Socket qw(:DEFAULT :crlf);

$/ = CRLF;

my $port = 20000;
my $false = 0;

my $sock = IO::Socket::INET->new( Proto     => 'tcp',
                                 LocalPort => $port,
                                 Listen    => SOMAXCONN,
                                 Reuse     => 1)
or die "can't create socket $! \n";

warn "Waiting for Incoming connection on $port \n";

while(!$false) {
  next unless my $session = $sock->accept;
  my $client = gethostbyaddr($session->peeraddr,AF_INET) || $session->
+peerhost;
  my $port = $session->peerport;
  warn "Connection from [$client, $port]\n";
  while(<$session>) {
    chomp;
    print $session "you typed $_ \n" ;
  }
  warn "Connection from [$client, $port] finished \n";
  close $session
}
close $sock;
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